1. Field of the Invention
The present invention relates to oil condition monitoring and in particular, to a sensor for detecting the presence and amount of water in oil or like substances. The invention also relates to a method of monitoring the presence of water in oil.
2. Description of the Related Art
Optical sensors have been used for oil condition monitoring for determining the presence of debris or otherwise monitor deterioration of a lubricant. Such devices may operate by shining light through a small gap and analysing the transmitted light with a suitable optical sensor. Alternative sensors may make use of scattering of light and may operate over different frequencies including outside of the visible range. Oil condition monitoring may be significant in providing feedback in advance of likely failure of a lubricant system. Action may be taken to perform maintenance or otherwise renew the lubricant.
Water in oil is of considerable concern to many mechanical systems. Minimal amounts of water may be absorbed by the oil during use, either from the atmosphere or by direct ingress of water into the system. As long as this water is in the absorbed state and the oil is unsaturated, the concern is minimal. Nevertheless, as the concentration of water approaches the saturation level, emulsified and free water may occur, which can be highly detrimental, especially if exposure is prolonged. In bearings, the incompressibility of water relative to the oil can result in disruption of the oil film leading to excessive wear. Just one percent water in oil can reduce the life expectancy of a bearing by as much as 90 percent. For ball or rolling element bearings, the localized pressure generated can cause spontaneous vaporization of the water, leading to erosive wear such as micropitting. The saturation level of water in oil may vary widely according to temperature and the type of oil and can range from 10 ppm to even 10000 ppm. Existing sensors capable of measuring the presence of water (free and dissolved) include capacitive sensors and Karl Fischer titration sensors. Both of these methods require considerable time for the sensor to reach equilibrium and are not ideal for rapidly changing conditions. Spectral analysis using Fourier Transform Infrared Spectroscopy (FTIR) has been used but is a relatively complex and costly procedure requiring calibration of the sensor relative to the spectrum produced with fresh oil.
A sensor has been proposed in co-pending application No PCT/EP2012/075437 by which water in oil can be conveniently detected and whereby calibration of the device is simplified. An alternative arrangement has been proposed in co-pending application No PCT/EP2012/075395 in which use of cost-effective LED's is proposed. The contents of these documents are incorporated herein by reference in their entirety. In both cases, the sensor has a gap for transmission of light between an emitter and receiver through a sample of the oil. The light passing through the oil from the emitter is detected at the receiver and a light signal representative of the light detected is analysed to determine an amount of signal fluctuation. A step change in the signal fluctuation is indicative of saturation of the oil.
Although the proposed devices have been found to function correctly, it would be desirable to both improve on their sensitivity to external factors and simplify their construction.